I start by lightly cooking the garlic cloves in olive oil until they brown. Then I add the tomatoes, followed by the wine, oregano, salt, basil, and cheese. I then slow cook everything for about an hour.

The sauce is turning out kind of watery though. How I can thicken it into a proper hearty Italian pasta sauce?

Are you draining the tomatoes? How much cheese do you normally add? At what temperature are you cooking? Is it in a pot or do you mean you "slow cook" in a crock pot? Also, cooking.stackexchange.com/questions/28448/… might be helpful.
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colejkeeneJan 25 '13 at 17:12

@lemontwist I think cooking it down longer than an hour would make for a better flavor than adding tomato paste, no? That stuff always taste so...bitter and plasticy to me.
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colejkeeneJan 25 '13 at 17:15

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You add an entire head of garlic to 16 oz of tomatoes, 1/2 cup wine and 1/4 cup olive oil? And here I thought I went heavy on garlic.
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Carey GregoryJan 26 '13 at 1:06

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@CareyGregory I love garlic. Good for the heart ;-)
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codeninjaJan 26 '13 at 4:55

Cooking down, definitely. Draining the tomatoes before cooking, absolutely. Breadcrumbs, that would be a little strange :-)
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SAJ14SAJJan 25 '13 at 17:31

Water is the sauce killer. If you can somehow avoid putting it in in the first place, that'd be nice, but otherwise, a good long simmer is the best method.
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SatanicpuppyJan 25 '13 at 17:36

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I wouldn't drain - there is flavour in the liquid. Boil off the water to intensify the flavour and thicken the sauce.
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slimJan 29 '13 at 12:38

@SAJ14SAJ (plain) breadcrumbs work great for a quick and dirty thickening of pretty much anything. I've even used them for mashed potatoes with too much cream. But I'll deny to the grave that if you ever ask me to my face.
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mikeTheLiarJan 29 '13 at 13:34

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bread in gazpacho is pretty traditional actually. A lot of sauces and soups from Spain are thickened with breads.
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BrendanJan 31 '13 at 21:41

There are a number of ways to help thicken sauce, but I think your first problem is you're not cooking it down enough. You want to simmer uncovered (sometimes a couple of hours), stirring often, to get it to thicken the way it sounds you want.

I tend to prefer fresh tomatoes to canned and avoid tomato paste (personally) to avoid a somewhat bitter flavor (until you pan fry the bitterness out of it first).

There is absolutely no way I would use a slurry on marinara as recommended in the number 2 suggestion at the linked article. The texture would just be wrong. I endorse cooking down.
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SAJ14SAJJan 25 '13 at 17:31

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@SAJ14SAJ I'm not sure it would make the best marinara, but then I don't think the breadcrumbs mentioned by user1190992 would be much better either. Cooking down is the best way, period, I was just offering time saving solutions if that was part of the issue.
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colejkeeneJan 25 '13 at 17:33

If your sauce doesn't taste watery and is just simply too thin, I'd suggest undercooking the pasta by a few minutes and letting it finish in the sauce (a handful of parmesan cheese doesn't hurt either).

A suggestion I would contribute is to swap the red wine for a smaller portion of red wine vinegar. It gives it a nice acidic flavor (you can balance this with some white sugar which will help it thicken even more, but I prefer my red sauces with more tang). It will also have less fluid overall.

If you don't mind a smoother sauce, like for pizza or the like, you can use an immersion or traditional blender to puree the sauce. This will make the sauce thicker and smoother. How thick, exactly, depends on how chunky the tomatoes are to begin with. If they are already fairly finely crushed, this won't help much. If they are more of a diced variety, this will help immensely. That being said, you are moving away from a traditional marinara if you puree too much. Maybe try removing some of the sauce, pureeing it, then stirring it back in. Old kitchen trick for thickening things without using cornstarch or a roux without necessarily compromising the original texture.